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Sunday, December 16, 2018

Astronomers Investigate a Nearby Magnetar in a Quiescent State

A team of European astronomers has investigated XTE J1810-197, one of the closest magnetars to our planet, during a period of its low activity. The study, which reveals the magnetar’s X-ray properties in detail, is available in a paper published December 7 on arXiv.org.

Magnetars are neutron stars with extreme magnetic fields - generally higher than 100 trillion Gauss (for comparison, the sun's magnetic field is only about 5 Gauss). Most magnetars are strongly variable exhibiting, at unpredictable times, large outbursts during which their X-ray flux increases up and then decays on a variety of timescales.

XTE J1810-197 was first spotted as a weak X-ray source by the ROSAT space telescope in 1993. Ten years later, a powerful outburst from this source was detected by the Rossi X-ray Timing Explorer (RXTE) satellite, and several short bursts during the initial phases of the outburst decay. All in all, subsequent observations of XTE J1810-197 allowed the astronomers to classify it as a magnetar.

The outburst of XTE J1810-197 lasted until early 2007 and since then the magnetar stays in a quiescent state. Given that XTE J1810-197 is one of the closest magnetars, its X-ray properties can be studied in detail even during its low activity. Therefore, a group of researchers Fabio Pintore of Institute of Space Astrophysics and Cosmic Physics of Milan, Italy, decided to conduct new X-ray observations of this star in order to learn more insights about it.

The observational campaign was carried out between June 2017 and April 2018, using XMM-Newton and Chandra spacecraft, as well as NICER instrument on the International Space Station (ISS).

“Here we first report the spectral and timing analysis of a new set of XMM-Newton, Chandra and NICER observations taken between June 2017 and April 2018 and then we use the whole dataset of the long quiescent period (2007–2018) to carry out a sensitive spectral analysis,” the astronomers wrote in the paper.

These observations allowed the researchers to find that the spin-down rate of XTE J1810-197 remained quite stable during the quiescent period. Pintore’s team also found that the magnetar does not showcase any significant spectral or flux variability, and that its spectrum can be described by the sum of two thermal components with temperatures of 0.15 and 0.3 keV, plus a power law component with photon index 0.6.

Furthermore, the researchers detected evidence in the phase-averaged spectrum of XTE J1810-197 of an absorption line centered at about 1.25 keV and with a width of approximately 0.1 keV. The authors of the paper suggest that this line is formed in a region located above the neutron star surface but is somehow displaced from the region where the pulse is produced.

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